1. Field of the Invention
The invention relates to a process for producing a premium, synthetic lubricant base stock produced from waxy, Fischer-Tropsch synthesized hydrocarbons. More particularly the invention relates to an isoparaffinic lubricant base stock produced by hydroisomerizing a waxy. paraffinic Fischer-Tropsch synthesized hydrocarbon fraction and catalytically dewaxing the hydroisomerate with a Pt/H-mordenite dewaxing catalyst.
2. Background of the Invention
Current trends in the design of automotive engines require higher quality crankcase and transmission lubricating oils having a high viscosity index (VI) and low pour point. While high VI""s have typically been achieved with the use of VI improvers as additives to the oil, additives are expensive and tend to undergo degradation from the high engine temperatures and shear rates. Processes for preparing lubricating oils of low pour point from petroleum derived feeds typically include atmospheric and/or vacuum distilling a crude oil to recover fractions boiling in the lubricating oil range, solvent extracting the lubricating oil fractions to remove aromatics and form a raffinate, hydrotreating the raffinate to remove heteroatom compounds and aromatics, followed by either solvent or catalytically dewaxing the hydrotreated raffinate to reduce the pour point of the oil. More recently it has been found that good quality lubricating oils can be formed from hydrotreated slack wax and Fischer-Tropsch wax.
Fischer-Tropsch wax is a term used to describe waxy hydrocarbons produced by a Fischer-Tropsch hydrocarbon synthesis processes, in which a synthesis gas feed comprising a mixture of H2 and CO reacts in the presence of a Fischer-Tropsch catalyst, under conditions effective to form hydrocarbons. U.S. Pat. No. 4,963,672 discloses a process for converting waxy Fischer-Tropsch hydrocarbons to a lubricant base stock having a high VI and a low pour point by sequentially hydrotreating, hydroisomerizing, and solvent dewaxing. A preferred embodiment comprises sequentially (i) severely hydrotreating the wax to remove impurities and partially convert the 1050xc2x0 F.+wax, (ii) hydroisomerizing the hydrotreated wax with a noble metal on a fluorided alumina catalyst, (iii) hydrorefining the hydroisomerate, (iv) fractionating the hydroisomerate to recover a lube oil fraction, and (v) solvent dewaxing the lube oil fraction to produce the base stock. European patent publication EP 0 668 342 A1 suggests a processes for producing lubricating base oils by hydrogenating and then hydroisomerizing a waxy Fischer-Tropsch raffinate, followed by dewaxing. The hydrogenating is performed without cracking to lower the hydroisomerization temperature and increase the catalyst life, both of which those skilled in the art know are adversely effected by the presence of oxygenates and heteroatoms in the waxy feed. EP 0 776 959 A2 recites hydroconverting Fischer-Tropsch hydrocarbons having a narrow boiling range, fractionating the hydroconversion effluent into heavy and light fractions and then dewaxing the heavy fraction to form a lubricating base oil having a VI of at least 150.
A premium, synthetic, isoparaffinic lubricant base stock having a high VI and a low pour point is made from a high purity, paraffinic, waxy Fischer-Tropsch synthesized hydrocarbon feed having an initial boiling point in the range of from 650-750xc2x0 F. (650-750xc2x0 F.+). by hydroisomerizing the feed and catalytically dewaxing the 650-750xc2x0 F.+ hydroisomerate with a dewaxing catalyst comprising a catalytic platinum component, and the hydrogen form of mordenite (hereinafter, xe2x80x9cPt/H-mordenitexe2x80x9d). By lubricant is meant a formulated lubricating oil, grease and the like. Fully formulated lubricating oils, made by forming an admixture of one or more lubricant additives and the base stock of the invention, have been found to perform at least as well as, and often superior to, formulated lubricating oils employing either a petroleum oil or PAO (polyalphaolefin) derived base stock. By 650-750xc2x0 F.+ is meant that fraction of the hydrocarbons synthesized by the Fischer-Tropsch process having an initial boiling point in the range of from 650-750xc2x0 F., preferably continuously boiling up to an end boiling point of at least 1050xc2x0 F., and more preferably continuously boiling up to an end point greater than 1050xc2x0 F. A Fischer-Tropsch synthesized hydrocarbon feed comprising this 650-750xc2x0 F.+ material, will hereinafter be referred to as a xe2x80x9cwaxy feedxe2x80x9d. By waxy is meant including material which solidifies at standard conditions of room temperature and pressure. The waxy feed also has a T90-T10 temperature spread of at least 350xc2x0 F. The temperature spread refers to the temperature difference in xc2x0 F., between the 90 wt. % and 10 wt. % boiling points of the waxy feed. The use of a dewaxing catalyst comprising Pt/H-mordenite in the process of the invention has been found produce higher yields of base stock at equivalent pour point, then is typically obtained with petroleum derived materials, such as hydrotreated slack wax.
Thus, the invention relates to a process for producing a high VI, low pour point lubricant base stock from a Fischer-Tropsch synthesized waxy feed by first (i) hydroisomerizing the waxy feed to form a hydroisomerate and then (ii) catalytically dewaxing the hydroisomerate to reduce its pour point by reacting it with hydrogen in the presence of a dewaxing catalyst comprising Pt/H-mordenite, to produce a dewaxate which comprises the base stock. The hydroisomerization is achieved by reacting the waxy feed with hydrogen in the presence of a suitable hydroisomerization catalyst and preferably a dual function catalyst which comprises at least one catalytic metal component to give the catalyst a hydrogenation/dehydrogenation function and an acidic metal oxide component to give the catalyst an acid hydroisomerization function. Preferably the hydroisomerization catalyst comprises a catalytic metal component comprising a Group VIB metal component, a Group VIII non-noble metal component and an amorphous alumina-silica component. Both the hydroisomerization and the dewaxing convert some of the 650-750xc2x0 F.+ hydrocarbons to hydrocarbons boiling below the 650-750xc2x0 F. range (650-750xc2x0 F.xe2x88x92). While this lower boiling material may remain in the hydroisomerate prior to dewaxing, it is removed from the dewaxat Removal is accomplished by flashing or fractionation. Dewaxing the entire hydroisomerate means that a larger dewaxing reactor is needed and more lower boiling material must be removed from the 650-750xc2x0 F.+ dewaxate, than if it was removed prior to dewaxing. The remaining 650-750xc2x0 F.+ dewaxate is typically fractionated into narrow cuts to produce base stocks of differing viscosity, although the entire dewaxate may be used as a base stock, if desired. By high VI and low pour point is meant that the entire 650-750xc2x0 F.+ dewaxate will have a VI of at least 110 and preferably at least 120, with a pour point less than xe2x88x9210xc2x0 C. and preferably less than xe2x88x9220xc2x0 C. Therefore, by lubricant base stock is meant all or a portion of the 650-750xc2x0 F.+ dewaxate produced by the process of the invention.
The dewaxing is conducted to convert no more than 40 wt. % and preferably no more than 30 wt. % of the 650-750xc2x0 F.+ hydroisomerate to 650-750xc2x0 F.xe2x88x92 material. In contrast to the process disclosed in U.S. Pat. No. 4,963,672 referred to above, due to the very low or nil concentration of nitrogen and sulfur compounds and the very low oxygenates level in the waxy feed, hydrogenation or hydrotreating is not required prior to the hydroisomerization and it is preferred in the practice of the invention that the waxy feed not be hydrotreated prior to the hydroisomerization. Eliminating the need for hydrotreating the Fischer-Tropsch wax is accomplished by the use of the relatively pure waxy feed, such as is produced by the slurry Fischer-Tropsch process with a catalyst comprising a cobalt catalytic component and, in a preferred embodiment, using a hydroisomerization catalyst resistant to poisoning and deactivation by any oxygenates that may be present.